Amplifier

ABSTRACT

An amplifier for a receiver circuit is disclosed. The amplifier has an input node (Vin) and an output node (Vout). It comprises a tunable tank circuit connected to the output node (Vout), a feedback circuit path connected between the output node (Vout) and the input node (Vin), and a tunable capacitor connected between an internal node of the feedback circuit path and a reference-voltage node. A receiver circuit and a communication apparatus is disclosed as well.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. application Ser. No.16/425,691 filed May 29, 2019, which claims the benefit of U.S.application Ser. No. 15/316,562 filed Dec. 6, 2016, (371(c) date) (nowU.S. Pat. No. 10,320,341 issued on Jun. 11, 2019), which is a 35 U.S.C.§ 371 national stage of international application PCT/EP2016/059791filed May 2, 2016. All of these earlier applications are herebyincorporated herein by reference in their entireties.

TECHNICAL FIELD

The present invention relates to an amplifier circuit.

BACKGROUND

Radio receiver circuits are used in many different applications, such ascellular communications. Signals received by a radio receiver circuitmay be relatively weak and need to be amplified. Hence, an amplifier istypically included in the radio receiver circuit. Such an amplifiershould not add too much noise to the received signal. Therefore, a socalled low-noise amplifier (LNA) is often used for this purpose.

Some existing radio communications systems, such as fourth generation(4G) and fifth generation (5G) cellular communications systems, the usedsignal bandwidth is often relatively large, such as tens or hundreds ofMHz. Furthermore, it should be possible to tune a receiver centerfrequency over a relatively large frequency range. One challenging taskin the design of receiver amplifiers, such as LNAs, is to achieve aninput impedance matching with enough frequency bandwidth. Suchrelatively wideband input impedance matching should preferably beobtained while at the same reaching sufficiently high performance interms of other parameters of the LNA, such as gain and frequencyselectivity.

SUMMARY

Embodiments of the present invention concern an amplifier, such as anLNA, for a receiver circuit, having a tunable tank circuit, such as anLC circuit, connected to an output node of the amplifier. Such a tankcircuit can provide a desired degree of frequency selectivity.Embodiments of the amplifier further comprise a feedback network betweenthe output node and an input node. Such a feedback network canfacilitate in providing input impedance matching. The inventor hasrealized that the phase of the output voltage, generated at the tankcircuit, changes relatively abruptly around the resonance frequency ofthe tank circuit. The relatively abruptly changed phase has an impact onthe feedback, and makes it challenging to meet input impedance matchingrequirements. For example, maximum gain of the amplifier and best inputimpedance matching may occur at different frequencies, which isundesired. Hence, some type of tuning is needed. The inventor hasrealized that a relatively efficient tuning can be obtained byconnecting a feedback circuit path of the feedback network between theoutput node and the input node, and by connecting a tunable capacitorbetween an internal node of the feedback circuit path and a referencevoltage node, such as ground or signal ground. An advantage of thisapproach is that the tunability can be obtained with components, such asthe tunable capacitor, with relatively low Q value. Such components arenormally easier and cheaper to manufacture than components with higher Qvalue.

According to a first aspect, there is provided an amplifier for areceiver circuit. The amplifier has an input node and an output node.The amplifier comprises a tunable tank circuit connected to the outputnode. Furthermore, the amplifier comprises a feedback circuit pathconnected between the output node and the input node. Moreover, theamplifier comprises a tunable capacitor connected between an internalnode of the feedback circuit path and a reference-voltage node.

In some embodiments, the feedback circuit path is a passive circuit.

The feedback circuit path may comprise a series connection of at leastone resistor and at least one capacitor.

In some embodiments, the at least one resistor is tunable. Thisfacilitates an even further degree of fine tuning of the inputimpedance.

In some embodiments, said at least one capacitor comprises a firstcapacitor, and said at least one resistor comprises a first resistorconnected between the output node and the first capacitor and a secondresistor connected between the first capacitor and the input node. Saidinternal node may be a node between the first capacitor and the secondresistor.

The amplifier may comprise a first transistor in common-sourceconfiguration. Furthermore, the amplifier may comprise a secondtransistor connected in a cascode configuration between the firsttransistor and the output node.

According to a second aspect, there is provided a differential amplifierfor a receiver circuit, comprising a first and a second amplifieraccording to any preceding claim. The first and second amplifier mayshare some components, such as the tank circuit.

According to a third aspect, there is provided a receiver circuitcomprising an amplifier according to the first aspect or a differentialamplifier according to the second aspect.

According to a fourth aspect, there is provided a communicationapparatus comprising the receiver circuit according to the third aspect.

The communication apparatus may be a wireless communication device for acellular communication network. The communication apparatus may be aradio base station for a cellular communication network.

Further embodiments are defined in the dependent claims. It should beemphasized that the term “comprises/comprising” when used in thisspecification is taken to specify the presence of stated features,integers, steps, or components, but does not preclude the presence oraddition of one or more other features, integers, steps, components, orgroups thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of embodiments of the inventionwill appear from the following detailed description, reference beingmade to the accompanying drawings, in which:

FIG. 1 illustrates a communication environment.

FIG. 2 illustrates a transceiver circuit.

FIGS. 3-7 shows circuit diagrams of amplifier circuits.

FIG. 8 illustrates implementation of a tunable resistor.

FIG. 9 illustrates implementation of a tunable capacitor.

FIG. 10 shows simulation results.

DETAILED DESCRIPTION

FIG. 1 illustrates an environment in which embodiments of the presentinvention may be employed. In FIG. 1, a wireless communication device 1is in wireless communication with a base station 2 of a cellularcommunication system. In FIG. 1, the wireless communication device 1 isillustrated as a mobile phone. However, this is only an example. Thewireless communication device may be any kind of device equipped withcellular communication capabilities, such as a table computer, laptopcomputer, cellular modem, or machine-type communication (MTC) device.The wireless communication device 1 and base station 2 are examples ofwhat in this disclosure is referred to as communication apparatuses. Itshould be noted that other communication apparatuses than cellularcommunication apparatuses, such as terminals and access points forwireless local area networks (WLANs), are possible as well within thescope of the present disclosure.

According to embodiments of the present disclosure, a communicationapparatus, such as those described above, comprises a receiver circuit.The receiver circuit may e.g. be part of a transceiver circuit. FIG. 2illustrates an example of such a transceiver circuit 10. In FIG. 2, thetransceiver circuit 10 comprises a receiver circuit 15, arranged to beconnected to an antenna 20. In FIG. 2, the receiver circuit 15 comprisesa radio frequency (RF) filter 25 arranged to be connected to the antenna20. In some embodiments, the RF filter 25 is a band-pass filter.Furthermore, in FIG. 2, the receiver circuit 15 comprises an amplifier30. Embodiments of the amplifier 30 are described in more detail below.The amplifier 30 may e.g. be what is commonly referred to as an LNA. InFIG. 2, the receiver circuit comprises a down-conversion mixer 35,driven by a local oscillator (LO) signal, connected to an output node ofthe amplifier 30. The down-conversion mixer 35 is configured to downconvert the RF signal output from the amplifier 30 to a basebandfrequency or intermediate frequency. Furthermore, in FIG. 2, thereceiver circuit 15 comprises a filter 40 arranged to filter outunwanted signal components from the down-converted signal output fromthe mixer 35. In some embodiments, the filter 40 is a low-pass filter.Moreover, in FIG. 2, the receiver circuit 15 comprises ananalog-to-digital converter (ADC) 45 configured to convert the filtereddown-converted signal output from the filter 40 to the digital domain.

As illustrated in FIG. 2, the transceiver circuit 10 may comprise adigital signal processor (DSP) 50, such as a baseband processor,configured to process the digital output signal from the ADC 45, e.g. torecover received data.

As also illustrated in FIG. 2, the transceiver circuit 10 may comprise atransmitter circuit 55, arranged to be connected to an antenna 60 fortransmitting RF signals. The DSP 50 may be configured to generate inputdata to the transmitter circuit 55.

The diagram of the receiver circuit 15 is merely an example used to putembodiments of the amplifier 30 in a context. Embodiments of theamplifier 30 may be used in other receiver architectures as well. Itshould be mentioned that embodiments of the amplifier 30 may beintegrated on an integrated circuit, e.g. together with some or all ofthe other components of the receiver circuit 15.

FIG. 3 illustrates an embodiment of the amplifier 30. In FIG. 3, theamplifier 30 has an input node V_(in) and an output node V_(out).Furthermore, it comprises a tunable tank circuit 100 connected to theoutput node Vow. Moreover it comprises a feedback network 110 betweenthe output node V_(out) and the input node V_(in). In order to provideamplification, an active element, such as a transistor, is generallyused in an amplifier. The embodiment of the amplifier 30 illustrated inFIG. 3 comprises a MOS (Metal-Oxide-Semiconductor) transistor 120 incommon-source configuration. In FIG. 3, the gate terminal of thetransistor 120 is connected to the input node V_(in). Other types oftransistors, such as bipolar junction transistors (BJTs) are possible aswell. Moreover, in FIG. 3, the amplifier 30 comprises a MOS transistor130 connected in a cascode configuration between the transistor 120 andthe output node V_(out). The gate terminal of the transistor 130 isconnected to a bias voltage node V_(b2). In some embodiments, thecascode transistor 120 may be omitted. Other embodiments may includemore than one cascode transistor.

As illustrated in FIG. 3, the amplifier 30 may comprise asource-degeneration inductor 140, connected between the source oftransistor 120. It may also comprise a biasing resistor 150 connectedbetween the input node V_(in) and a bias voltage node V_(b1). FIG. 3also illustrates some reactive components, such as an inductor 160 and acapacitor 170, connected in series with the inductor 160 between theinput node V_(in) and preceding components, such as the filter 25 (FIG.2). Such reactive components 160, 170 facilitate the input impedancematching for the amplifier 30.

The inventor has realized that the phase of output voltage, generated atthe tank circuit, changes relatively abruptly around the resonancefrequency of the tank circuit. The relatively abruptly changed phase hasan impact on the feedback, and makes it challenging to meet inputimpedance matching requirements. It may be particularly challenging inapplications with relatively high bandwidth, such as in the GHz range,and with relatively high carrier frequencies, such as several tens ofGHz, e.g. as will likely be used for 5G systems in the future. Forexample, maximum gain of the amplifier and best input impedance matchingmay occur at different frequencies, which is undesired. Hence, some typeof tuning is needed. FIG. 4 illustrates an advantageous implementationof the feedback network 110 provided by the inventor. It comprises afeedback circuit path 200 connected between the output node V_(out) andthe input node V_(in). Furthermore, it comprises a tunable capacitor 210connected between an internal node of the feedback circuit path 200 anda reference-voltage node, such as ground or signal ground.

The tunable capacitor 210 facilitates compensation of the relativelyabruptly changed phase of the output voltage around the resonancefrequency of the tank circuit 100. It enables tuning of the amplifier 30such that, for instance, the maximum gain of the amplifier 30 and thebest input impedance matching of the amplifier 30 can be tuned, infrequency, to occur at substantially the same frequency. Furthermore,simulations have shown that the capacitor 210 can be implemented with arelatively low Q value, while still providing this desired tunability.This is advantageous, since the tunability can be obtained at arelatively low cost with relatively small components.

The inventor has further realized that the feedback circuit path 200 canbe implemented as a passive circuit. Using a passive feedback circuitpath makes it relatively easy to obtain a combination of relatively highgain and stability, which can be a very challenging design goal if anactive feedback circuit path would be used. Furthermore, a passivefeedback circuit typically does not require any complex biasing circuit.Nevertheless, a desired input impedance matching can be obtained also inembodiments with active components in the feedback circuit path.

For example, the feedback circuit path can be implemented using a seriesconnection of at least one resistor and at least one capacitor. This isillustrated in FIG. 5 with an embodiment wherein said at least onecapacitor comprises a first capacitor 240, and said at least oneresistor comprises a first resistor 220 connected between the outputnode V_(out) and the first capacitor 240 and a second resistor 230connected between the first capacitor 240 and the input node V_(in).Furthermore, in FIG. 5, the internal node, to which the capacitor 210 isconnected, is a node between the first capacitor 240 and the secondresistor 230.

As illustrated in FIG. 5, the at least one resistor (e.g., 220, 230) canbe tunable as well. This provides a further degree of tunability tofacilitate tuning of the input impedance implemented with relativelycheap and small components.

FIG. 6 illustrates an embodiment of the tunable tank circuit 100. As canbe seen from FIG. 6, the tunable tank circuit 100 may comprise aparallel LC circuit, comprising a capacitor 250 connected in parallelwith an inductor 260. As is further illustrated in FIG. 6, thetunability of the tank circuit 100 can be provided by making thecapacitor 250 tunable, whereby the resonance frequency of the tankcircuit 100 can be tuned. Preferably, the resonance frequency of thetank circuit is tuned to around the center frequency of a desired signalfrequency band.

As is further illustrated in FIG. 6, the tank circuit 100 may compriseanother inductor 270 magnetically coupled to the inductor 260 with amutual inductance M. Such a solution can provide a desired overallinductance of the tank circuit with smaller inductor coils compared withembodiments where the inductor 260 is used alone, without the additionalinductor 270. Inductors 260 and 270 forms a primary and secondarywinding, respectively, of a transformer. In an example embodiment, nodesS1 and S2 of inductor 270 are used to drive subsequent stages in thereceiver circuit 15, whereas node S0 is connected to a bias voltagenode.

According to some embodiments, two of the amplifiers 30 are combinedinto a differential amplifier. Such a differential amplifier can, ofcourse, be used in a receiver circuit, e.g. as a differential LNA. Anexample of such an embodiment is illustrated in FIG. 7. The embodimentof the differential amplifier illustrated in FIG. 7 can be seen as adifferential version of the embodiment of the amplifier 30 illustratedin FIG. 6. Components 110 a, 120 a, 130 a, 140 a, 150 a, 160 a, 170 a,200 a, and 210 a, correspond to the components 110, 120, 130, 140, 150,160, 170, 200, and 210 (FIG. 6) of a first one of the two amplifiersmaking up the differential amplifier. Similarly, components 110 b, 120b, 130 b, 140 b, 150 b, 160 b, 170 b, 200 b, and 210 b, correspond tothe components 110, 120, 130, 140, 150, 160, 170, 200, and 210 (FIG. 6)of a second one of the two amplifiers making up the differentialamplifier. In FIG. 7, the differential amplifier comprises a shared tankcircuit 100. However, in other embodiments, there may be separate tankcircuits for the two amplifiers making up the differential amplifier.The nodes V_(ina) and V_(inb) make up a differential input port.Similarly, the output nodes V_(outa) and V_(outb) make up a differentialoutput port.

FIG. 8 illustrates a possible implementation of a tunable resistor 400.It illustrates that a tunable resistor 400 can be implemented with anumber of parallel-connected switchable resistors, each comprising aresistor 410-i in series with a switch 420-i. By selecting which of theswitches 420-i are closed and which are open, the total resistance ofthe tunable resistor 400 can be tuned to a desired value, as would bereadily understood by a person skilled in electronic circuit design. Theswitches 420-i can e.g. be controlled with a digital control word, whereeach bit of the control word controls a unique one of the switches420-i. Any of the tunable resistors discussed in this disclosure can bedesigned in this way.

In a similar way, FIG. 9 illustrates a possible implementation of atunable capacitor. It illustrates that a tunable capacitor 500 can beimplemented with a number of parallel-connected switchable capacitors,each comprising a capacitor 510-i in series with a switch 520-i. Byselecting which of the switches 520-i are closed and which are open, thetotal capacitance of the tunable capacitor 500 can be tuned to a desiredvalue, as would be readily understood by a person skilled in electroniccircuit design. The switches 520-i can e.g. be controlled with a digitalcontrol word, where each bit of the control word controls a unique oneof the switches 520-i. Any of the tunable capacitors discussed in thisdisclosure can be designed in this way.

FIG. 10 illustrates how the phase of the feedback current, from thefeedback network 110, into the input node V_(in) varies with frequencyand capacitance C of the capacitor 210 according to a simulationexample. In the simulation example, the circuit topology of FIG. 5 hasbeen used, and an RF AC voltage source with a 50Ω output resistance hasbeen used to feed the left node of the capacitor 170. The quantitativebehavior of the curve naturally depends on component parameter valuesfor the circuit components. Selection of such component parameter valuesfor a given requirement specification, e.g. based on circuit simulation,is considered a routine task for a person skilled in the art of analogRF circuit design and is not discussed herein in any further detail. Aninteresting observation, in the context of this disclosure, that can bemade from FIG. 10 is the qualitative behavior of the curves. One of thecurves is labeled C=0. This case, where the capacitance C of thecapacitor 210 is 0, corresponds to a case where the capacitor 210 isabsent. It can be observed that there is a relatively abrupt phasevariation around the resonance frequency to the tank circuit 100, whichin this simulation is around 30 GHz. This relatively abrupt phasevariation makes it difficult to achieve a good input impedance matchingover more than a relatively narrow frequency range. Furthermore, thebest input impedance matching may occur at another frequency than theresonance frequency of the tank circuit 100, in particular inapplications where the receiver circuit 15 is tunable to differentfrequency bands and, therefore, the tank circuit 100 is tunable todifferent resonance frequencies. As the value of C is increased, it canbe observed that the phase variation is reduced, which enables inputimpedance matching over a wider frequency range. Furthermore, it ispossible to tune the value of C such that the best, or at least anadequately good, input impedance matching is provided at the centerfrequency of the tank circuit 100. As mentioned above, such tuning canbe obtained using tunable components with relatively low Q value.

The present invention has been described above with reference tospecific embodiments. However, other embodiments than the abovedescribed are possible within the scope of the disclosure. The differentfeatures of the embodiments may be combined in other combinations thanthose described.

The invention claimed is:
 1. An amplifier for a receiver circuit, theamplifier having an input node (V_(in)) and an output node (V_(out)),comprising: a tank circuit coupled to the output node (V_(out)); acircuit path comprising first and second nodes and a third node situatedbetween the first and second nodes, wherein the circuit path is coupledbetween the output node (V_(out)) and the input node (V_(in)); and atunable capacitor coupled between the third node of the circuit path anda reference-voltage node, wherein the coupled circuit path is a passivecircuit.
 2. The amplifier according to claim 1, wherein the circuit pathcomprises a series connection of at least one resistor and at least onecapacitor.
 3. The amplifier according to claim 2, wherein the at leastone resistor is tunable.
 4. The amplifier according to claim 2, whereinsaid at least one capacitor comprises a first capacitor, said at leastone resistor comprises a first resistor coupled between the output node(V_(out)) and the first capacitor and a second resistor coupled betweenthe first capacitor and the input node (V_(in)).
 5. The amplifieraccording to claim 4, wherein the third node of the circuit path isbetween the first capacitor and the second resistor.
 6. The amplifieraccording to claim 1, comprising a first transistor in common-sourceconfiguration.
 7. The amplifier according to claim 6, comprising asecond transistor coupled in a cascode configuration between the firsttransistor and the output node (V_(out)).
 8. A differential amplifierfor a receiver circuit, comprising a first and a second amplifier, eachaccording to claim
 1. 9. A receiver circuit comprising an amplifieraccording to claim
 1. 10. A communication apparatus comprising thereceiver circuit according to claim
 9. 11. The communication apparatusaccording to claim 10, wherein the communication apparatus is a wirelesscommunication device for a cellular communication network.
 12. Thecommunication apparatus according to claim 10, wherein the communicationapparatus is a radio base station for a cellular communication network.13. A receiver circuit comprising the differential amplifier of claim 8.14. The amplifier according to claim 6, comprising an inductor coupledto the source terminal of the first transistor.
 15. The amplifieraccording to claim 1, wherein the tank circuit has a tunable centerfrequency.
 16. The amplifier according to claim 1, wherein: the inputnode is coupled to an output node of another circuit of the receivercircuit; and the tunable capacitor is configured to be tuned to causethe amplifier to have an input impedance that matches an outputimpedance of said another circuit at each of a plurality of centerfrequencies of the receiver circuit.
 17. The receiver circuit accordingto claim 9, wherein: the input node is coupled to an output node ofanother circuit of the receiver circuit; and the tunable capacitor isconfigured to be tuned to cause the amplifier to have an input impedancethat matches an output impedance of said another circuit at each of aplurality of center frequencies of the receiver circuit.
 18. Thereceiver circuit according to claim 13, wherein in one or both of thefirst and second amplifiers: the input node is coupled to an output nodeof another circuit of the receiver circuit; and the tunable capacitor isconfigured to be tuned to cause the amplifier to have an input impedancethat matches an output impedance of said another circuit at each of aplurality of center frequencies of the receiver circuit.